High gate leakage current of nitrided silicon dioxide and depletion effect of polysilicon gate electrodes limit the performance of conventional silicon oxide based gate electrodes in metal oxide semiconductor field effect transistors (MOSFETs). High performance devices for an equivalent oxide thickness (EOT) less than 1 nm require high dielectric constant (high-k) gate dielectrics and metal gate electrodes to limit the gate leakage current and provide high on-currents. The high-k gate dielectrics have demonstrated improvement in short channel effects in transistors having a gate length less than 65 nm compared with conventional silicon oxide based gate dielectrics.
A high-k dielectric material needs to provide good electrical stability, that is, the amount of charge trapped in the high-k dielectric material needs to remain at a low level even after extended operation of a transistor. The high-k dielectric material needs to be scalable, that is, provide an acceptable level of leakage and an acceptable level of electron and hole mobility at a reduced thickness, e.g., less than 1 nm. While the mechanisms for degradation of mobility associated with thin high-k dielectric materials are not fully understood, it is generally believed that trapped charge scattering and/or phonon scattering are primary causes.
In view of the above, there exists a need for a MOSFET structure providing advantageous properties of conventional gate dielectrics and high-k gate dielectrics and methods of manufacturing the same.
Particularly, there exists a need for a MOSFFET structure having limited gate leakage as provided by high-k gate dielectrics and free of degradation of mobility that is typically associated with thin high-k gate dielectrics, and methods of manufacturing the same.
Further, modulation of work function of the gate electrode along a channel between a source and drain of a MOSFET may enhance device performance by increasing the on-current of the MOSFET. Specifically, a gate electrode of an n-type MOSFET may employ a first material having a first work function at a value less than the middle of the band gap of the channel material near the source, and a second material having a second work function at a value greater than the middle of the band gap of the channel material near the drain. Likewise, a gate electrode of a p-type MOSFET may employ a third material having a third work function at a value greater than the middle of the band gap of the channel material near the source, and a fourth material having a fourth work function at a value greater than the middle of the band gap of the channel material near the drain.
Therefore, there exists a need for a MOSFET structure having a graded work function across a channel and methods of manufacturing the same.
Specifically, there exists a need for a MOSFET structure having a gate electrode containing a first material and a second material, wherein the first material has a work function at a value closer to a conduction band edge of a channel material than a valence band of the same, and wherein the second material has a work function at a value closer to a valence band edge of a channel material than a conduction band edge of the same, and methods of manufacturing the same.